The unit tests for reviewboard.accounts has grown too large to
properly maintain. Some tests were in classes they didn't belong in, and
many tests were missing. Merges also became more difficult.

This change splits the test suite into a file per suite. The code for
the tests have not changed, with the exception that the sandbox tests
for auth backends have been split into two suites (one for each form
that's actually being tested in that suite), and some tests being pulled
out of improper test suites and into proper ones (probably in the wrong
places due to merges and user-contributed code).